1. Field of the Invention
The present invention relates to a process for prevention or reduction of discoloration of sodium or zinc pyrithione.
2. Description of the Prior Art
Zinc pyrithione [also known as zinc pyridine-2-thiol-N-oxide or bis[1-hydroxy-2(H) pyridinethionato]-zinc] is an excellent biocide. It has been employed as a broad-spectrum antimicrobial agent and preservative in metal working fluids, plastics, and cosmetics. Its principal uses are as an antidandruff agent in hair products or as a preservative in various cosmetics and toiletries. Sodium pyrithione [also called the sodium salt of 1-hydroxy-2-pyridinethione, sodium pyridine-2-thiol-N-oxide, or 2-pyridinethiol-1-oxide, Na salt] is also employed as a preservative in various cosmetics and toiletries.
Zinc pyrithione may be made by reacting 1-hydroxy-2-pyridinethione or a soluble salt thereof with a zinc salt (e.g., ZnSO.sub.4) to form a zinc pyrithione precipitate. See U.S. Pat. No. 2,809,971, which issued to Bernstein and Losee on Oct. 15, 1957. Generally, the sodium pyrithione is employed as the precursor of zinc pyrithione.
Since the esthetics of cosmetics and toiletries normally require certain desirable colors, and the formulators of such products go to great lengths to achieve specific color effects, any ingredient which varies very much from white or colorless may make the colorant formulators' task very difficult.
Sodium pyrithione and zinc pyrithione products occasionally have problems with meeting such strict color specifications. It is believed that such discoloration results from the oxidation of unwanted traces of contaminants during processing of the sodium pyrithione solutions. One method of removing these contaminants is to carry out multi-step purification processes with such solutions. This is costly and adds extra processing steps. Accordingly, there is a need for a better method of preventing or removing discoloration of sodium or zinc pyrithione.